gRPC can efficiently handle Bidirectional streaming communication, which is beneficial for services that need to maintain a persistent connection. Bidirectional streaming allows both the client and server to send and receive data simultaneously, making it suitable for real-time communication scenarios.

To make API keys more secure during transmission, it's essential to use HTTPS (SSL/TLS) to encrypt the communication. Transmitting keys in plain text, via email, or in public chat channels can expose them to potential attackers. HTTPS provides secure end-to-end encryption, making it significantly more difficult for malicious parties to intercept and misuse API keys.

GraphQL subscriptions are used to get real-time updates when an event or data change occurs on the server. This can include events like new data being added, data updates, or deletions. GraphQL subscriptions enable clients to receive real-time updates without the need for continuous polling.

Designing a RESTful API for a banking application requires a strong focus on security and reliability. Using HTTPS, implementing robust authentication, authorization, and encryption mechanisms are essential to protect sensitive financial data. Minimizing error handling and returning detailed error messages can pose security risks. Storing sensitive data in plain text is a security vulnerability.

A characteristic feature of gRPC is that it supports multiple programming languages, making it suitable for building cross-language and cross-platform applications. It uses Protocol Buffers (protobufs) for data serialization and supports asynchronous communication, which can improve application performance.

Web APIs act as a bridge between "Clients" and allow them to communicate and exchange data. Clients can be web applications, mobile apps, or other software that make requests to servers, which provide data and services. This interaction allows for the exchange of information and functionality between different systems, making it a fundamental aspect of modern web development.

To ensure the reliability of an API, it's important to implement Monitoring Tools that can detect and alert on potential issues. Monitoring tools track the performance and health of an API in real-time, helping to identify and respond to issues such as downtime, slow responses, or errors that can affect the end-users.

OpenID Connect returns an ID Token, which contains claims about the authentication of an end-user by an Authorization Server. This ID Token is a JSON Web Token (JWT) and includes information about the authenticated user, such as their user ID and authentication timestamp. It is a critical component of user authentication in OpenID Connect.

Performance testing ensures that an API can handle a specific load within acceptable response times. It measures how quickly the API responds to requests, ensuring that it meets performance expectations.

API authentication is a process that ensures that the client sending requests to the server is authorized. It verifies the identity and permissions of the client making API requests.